In general, there are inertial measurement units which utilize stored angular momentum to obtain angular velocity such as rate sensors and rate gyros. The purpose of the inertial measurement unit is to measure the motion of a body in inertial space and it may be utilized in guidance systems, auto pilots, or stable platform maintenance.
The problems of the stored angular momentum devices are centered around the size of the device, the sensitivity of the devices due to temperature changes, and ruggedization of the devices. Also, translational acceleration data is not available directly from these devices.
As will be described, the present processor processes signals from an inertial measurement unit of the type described in U.S. Patent Application, Ser. No. 528,243, now abandoned, filed Nov. 29, 1974 by Theodore Mairson and assigned to the assignee hereof. This inertial measurement unit utilizes accelerometers on a spinning rotor, the outputs of which are processed so that both angular velocity and translational velocity vectors are available from one rugged precise device. This uniquely specifies the inertial parameters of the case of the instrument.
It is the purpose of the present invention to provide a processor which, in combination with the above inertial measurement unit, not only provides as an output the angular velocity of the inertial measurement unit, but also provides as an output the translational acceleration of the inertial measurement unit in three orthogonal directions. It is a feature of the subject processor that the angular velocity and translational acceleration of the case in which the rotor is mounted can be determined directly from linear combinations of the outputs of the accelerometers utilized.
In order to utilize linear combinations of accelerometer outputs, it is a further feature of the subject invention that certain "dynamic" variables are utilized. The word "dynamic" is used in the sense of relating to the field of dynamics of moving bodies. The "dynamic" variables chosen have two important properties: first, they uniquely define the motion of the inertial measurement unit in inertial space; and secondly, these dynamic variables are derived through simple linear combinations of transducer outputs. These dynamic variables which have these properties are defined in equations 17-23 hereinafter. Since simple linear combinations yield all the relevant information about the motion of the inertial measurement unit, signal processing is greatly simplified.
In order to be able to perform the above signal processing, it is important that one of the accelerometers have a sensitive axis parallel to that of the rotor and that this axis be off-set therefrom. With utilization of such an accelerometer, it is now possible to obtain not only the signed magnitudes of the components of the angular velocity vector, but to correct these magnitudes by means of a combining network and measurements made by on-axis transducers.
While the equations which govern the physical phenomenon at the points at which acceleration measurements are made have been described in the past, it is important that by selecting particular combinations of the outputs of the transducers used to generate the above-mentioned dynamic variables, linear combinations of the transducer outputs lead directly to vector components. This eliminates complicated processing. This same analysis of the primary equations also leads to matrices in which cross-coupling, position and alignment errors are easily identified and corrected for by the formation of inverse combining matrices. Not only is initial calibration easily accomplished by changing matrix value, but the number of measurements necessary in order to establish the proper calibration is also minimized.
In the one rotor embodiment, four linear accelerometers are used, with three used for a.c. processing and one used to obtain a d.c. measurement of the velocity of the rotor in inertial space. With the parallel/offset transducer used alone, errors would exist due to errors in position, errors in alignment and errors due to cross coupling. With the addition of an accelerometer on the spin axis and having a sensitive axis normal to it, it is possible to correct for errors in translational acceleration caused by errors in position and alignment, and cross coupling. By adding two such transducers with sum and difference processing, it is possible to correct both for translational acceleration errors and errors in angular velocity due to the above factors.
In a second embodiment two rotors are utilized, with one rotor being orthogonally oriented with respect to the other rotor but in the same plane. Each rotor carries only three transducers, with the transducers on one rotor making possible the computation of the speed of the other rotor in inertial space. The reason for the utilization of the two rotor embodiment is that all measurements are taken at the rotational frequency, thereby precluding the necessity of making a d.c. measurement. For complete redundancy, in a third embodiment (not shown), a rotor may be placed in a third mutually orthogonal direction.
In any case, what has been provided is a processor for a small precise rugged unit which provides all necessary inertial measurements by simple processing techniques utilizing linear combinations of transducer outputs.
The subject inertial measurement unit and signal processor may be used anywhere inertial instruments are required such as autopilots, etc., and especially is useful in optically guided artillery shells which utilize canards to guide the shell. In this case, the shell itself forms the rotor and the despun canards form the case.
It is therefore an object of this invention to provide a signal processor for processing signals from an inertial measurement unit in which the processor utilizes a unique set of dynamic variables which describe the movement of the unit in terms of linear combinations of transducer outputs.
It is another object of this invention to provide a system of locating transducers within a rotating body such that inertial measurements are easily obtained through linear processing of the outputs of the transducers;
It is a further object of this invention to provide the combination of a new rugged inertial measurement unit and new signal processor in which angular velocity and translational acceleration vectors for the unit are easily obtained.
It is a still further object of this invention to provide a method and apparatus for processing signals from an inertial measurement unit which employs a linear combining matrix;
It is still another object of this invention to provide an all a.c. signal processing system for an inertial measurement unit in which the measuring of d.c. voltages is eliminated, while at the same time permitting the resolution of the angular velocity vector and the translational acceleration vector of the case housing the spinning rotor of the inertial measurement unit.
It is a still further object of this invention to provide a conveniently adjusted combining matrix and processing unit for an inertial measurement unit.
These and other objects, etc., will be better understood in connection with the following description of several embodiments taken on conjunction with the following drawings: